FIG. 12 shows a block diagram of a conventional communication device including conventional antenna switch module 80. Conventional antenna switch module 80 will be described with FIG. 12. In FIG. 12, the communication device includes antenna switch module 80, transmission part 41, reception part 42, filter 83, antennas 44 and 45, and capacitors C81 to C84.
Conventional antenna switch module 80 includes antenna switch circuit 87 and filter 86. Signals from transmission part 41 are inputted to filter 86 through capacitor 81. Filter 86 passes fundamental frequencies and removes unnecessary signals. Signals outputted from filter 86 are emitted from antenna 44 or 45 selected by antenna switch circuit 87, after passing through capacitor C83 or C84.
On the other hand, signals which are received by antenna 44 or 45 selected by antenna switch circuit 87 and then are passed through capacitor C83 or C84 are inputted to filter 83 through capacitor C82. Filter 83 removes unnecessary signals from the received signals and outputs them to reception part 42. Reception part 42 demodulates the signals from filter 83.
Filter 86 included in antenna switch module 80 will be described with reference to FIGS. 13 to 16. FIG. 13 shows the structure of the filter for the conventional antenna switch module. In FIG. 13, capacitors C94 connected to the ground are open circuit to low frequency components and are short circuit to high frequency components. Inductor L94 connected in series with capacitors C94 are short circuit to low frequency components and are open circuit to high frequency components. Thus, the filter shown in FIG. 13 is a low pass filter, which passes low frequency components only.
The low pass filter shown in FIG. 13 has a large circuit size because rapid attenuation can be achieved only by a large number of stages. Alternatively, rapid attenuation can be achieved by a few number of stages when the constant of each element of this filter is determined in such a manner as to make the filter a Chebychev low pass filter. However, it is difficult for this filter to achieve wideband filtering at low loss because the filter has ripples in the passband. Another possible structure is achieved by the use of distributed constant lines. In this case, however, when a certain frequency is reached, inductive distributed constant lines are changed to capacitive distributed constant lines, whereas capacitive distributed constant lines are changed to inductive distributed constant lines. The input impedance greatly changes depending on the frequency, thereby sometimes causing the filter to lose its function as a filter.
In view of this situation, a polarized low pass filter shown in FIG. 14 has been contrived. FIG. 14 shows the structure of this filter for the conventional antenna switch module. FIG. 15 shows frequency characteristics of this filter for the conventional antenna switch module. The principle of operation of the filter will be briefly described with FIGS. 14 and 15.
Polarized low pass filter 90 includes LC series circuits 96, 98 and 99. As shown in FIG. 15, the attenuation band in the frequency characteristics of filter 90 has three poles: a first pole of 13.2 KHz; a second pole of 15.4 KHz and a third pole of 25.3 KHz. Here, for example, decreasing the resonant frequency of LC series circuit 98, that is, the second pole frequency can reduce the interval between the first pole and the second pole. Reducing the interval between these poles results in an increase in the amount of attenuation between the poles.
As a filter for the conventional antenna switch module, one example of the aforementioned polarized low pass filter is disclosed in Japanese Patent Unexamined Publication No. 61-77408.
Another known filter for an antenna switch module is a notch filter with little filtering loss. A notch low pass filter can be formed of a combination of a plurality of ¼ wavelength open stubs and ½ wavelength terminated stubs. FIG. 16 shows frequency characteristics when antenna 44 or 45 is seen from transmission part 41 in a case where filter 86 of the conventional antenna switch module is a notch filter. Fundamental frequencies are from F1=4.9 GHz to F2=5.85 GHz. Second harmonic frequencies are from F3=9.8 GHz to F4=11.7 GHz. Third harmonic frequencies are from F5=14.7 GHz to F6=17.55 GHz. The frequencies of the attenuation poles are set so as to attenuate the second and third harmonics.
In such a conventional structure, the second harmonic frequencies have small attenuation at other than the two attenuation poles F3 and F4, and the third harmonic frequencies have small attenuation at other than the two attenuation poles F5 and F6. In other words, the impedance when the output side of filter 86 is seen from the input side, and the impedance when the input side is seen from the output side both approach 50 ohms. On the other hand, impedance Z813 when terminal T83 connected with capacitor C83 is seen from terminal T81 of antenna switch circuit 87, which is connected with filter 86 is about 50 ohms at the fundamental frequencies. Impedance Z842 when terminal T82 connected with capacitor C82 is seen from terminal T84 connected with capacitor C84 is also about 50 ohms at the fundamental frequencies. On the other hand, impedance Z814 when terminal T84 is seen from terminal T81 and impedance Z832 when terminal T82 is seen from terminal T83 are open circuit. However, as the frequency gets higher, the capacity component and induction component of the package and terminals of the PIN diode become influential. This causes impedances Z813, Z842, Z814 and Z832 to change their values. More specifically, impedances Z813 and Z842 have values close to open circuit at the harmonic frequencies ranging from 4.9 GHz to 5.85 GHz, and impedances Z814 and Z832 have values close to 50 ohms. As a result, at the harmonic frequencies, the impedance Z813 when filter 86 is seen from terminal T81, and impedance Z814 when antenna switch circuit 87 is seen from terminal T81 can be complex conjugates of each other. This may deteriorate the amount of attenuation at the attenuation poles or cause a rebound phenomenon between the attenuation poles, thereby making it impossible to have enough attenuation.